1. Field of the Invention
The present invention relates to an inorganic nanoparticle of a skutterudite compound that can be used for example for a thermoelectric conversion material and the like.
2. Description of the Related Art
The method for producing an inorganic nanoparticle can roughly be classified into the solid phase method, the liquid phase method and the gas phase method. As a method for producing an inorganic nanoparticle having a narrow particle size distribution, the liquid phase method and the gas phase method as the build up process are effective. In particular, the liquid phase method is excellent, since a chemically homogeneous composition can be provided in the case of two or more kinds of the constituent elements. The liquid phase method can be classified into the coprecipitation method, the reverse micelle method, and the hot soap method. Among them, the hot soap method has been recently attracted attentions as a method for producing an inorganic nanoparticle because it can provide a relatively high crystal property.
As the examples of inorganic nanoparticles produced using the hot soap method, the semiconductor materials showing the light emitting characteristics, such as CdSe, CdS and ZnS can mainly be presented (see the J. Am. Chem. Soc., 115, p. 8706-8715 (1993)). These semiconductor materials are known to show the sharper light emitting characteristics by the quantum effect by processing the same into nanoparticles. Application thereof is discussed mainly in the label field in bio and the display field.
Moreover, as to the magnetic recording material, in order to realize a high density recording by miniaturizing the size, synthesis of the inorganic nanoparticles using the hot soap method has been studied. As the magnetic recording materials produced by the hot soap method so far, specifically, Co, FePt, CoPt3, and the like can be presented (see the Science, 1989, p. 287 (2000)).
On the other hand, although the thermoelectric conversion material is a material for directly converting the heat to the electricity, recently, a theory that the thermoelectric conversion efficiency can be increased by the quantum effect has been made public. Thereafter, the material study has been conducted, and it is known that a high performance of ZT=2.4 by the super lattice structure and ZT=2 by the quantum dot super lattice is provided. Since the high performance by the quantum effect can be expected also by processing the thermoelectric conversion material into the nanoparticles, synthesis of the inorganic nanoparticles using the hot soap method is also studied. As the thermoelectric conversion materials produced so far by the hot soap method, Bi, PbSe, and the like can be presented (see the Mat. Res. Soc. Symp., Proc., 691, G10. 2 (2002)).
The skutterudite compound is a material showing a high thermoelectric conversion performance. Particularly owing to the high mobility, deterioration of an electric conductivity due to interface scattering anticipated at the time of coupling the inorganic nanoparticles is expected to be restrained. The skutterudite compound is a compound represented by MX3 (M: Co, Rh, Ir, X: P, As, Sb). Synthesis of CoSb3 by hydrogen reduction after synthesizing inorganic nanoparticles including the oxides of co and Sb by the coprecipitation method has been reported so far (See Nanostructured Films and Coatings, p. 149-156 (2000)). However, since the particle size of the CoSb3 particles synthesized by this method is 200 nm or more and it should be at least 100 nm or less for realizing the quantum effect, it is difficult to improve the performance. Moreover, a problem is involved in that whether or not the CoSb3 particles are reduced to the center.
Moreover, synthesis of the inorganic nanoparticles of a skutterudite compound having a 100 nm or less particle size has not been reported.